DrByte, Thank you for that~!I got the API signatures this morning :) ..but I got other problems while I try to test it.
Firstly, I use PAYPAL Express as the sole payment method, but there is no "pay without a paypal account"function...ppl have to open a paypal a/c to pay

I research the paypal.com and zen cart admin area but couldn't find the related setting...
Secondly, if I do a test transaction, how much I should pay paypal for the transaction fee?0.3?

I know there is a sand box method for testing, but it seems so inconvenience.
Last, when I use a custom temple, I was unable to log in as a customer in the store... I did a research on this forum, and found a tutorial related to my problem ""There was a security error when trying to login"" on
https://www.zen-cart.com/tutorials/i...hp?article=312
But after read that I still don't know how to fix it.Should I insert some codes or php files to my custom template folder or not..
Any help could be really appreciate, Thanks~!